1. Field of the Invention
The present invention relates to a high-frequency heating apparatus such as a microwave oven for heating foods or fluid, or more in particular to a power supply unit for a high-frequency heating apparatus comprising a frequency converter for converting the power of a commercial power supply or a battery into high-frequency power.
2. Description of the Related Art
As is well known, many power supply units for microwave ovens include a power supply circuit built around an iron resonance type of transformer 2, as shown in FIG. 1. A magnetron 5 normally has an operating voltage of 3 to 4 kv,, and therefore the circuit voltage on the secondary of the boosting transformer 2 is dangerously high. A housing 6 for the microwave oven, therefore, is always grounded as indicated at 7.
In the above-mentioned configuration of the conventional microwave oven, in the case where a high-voltage winding 8 and a primary winding 9 come in mixed contact with each other due to some fault as shown in FIG. 1 (such as when a point P comes into contact with a point Q), a grounding line 10, if provided, would maintain the housing 6 at ground potential and cause a fuse 11 to be burnt out, thus keeping safe a human being touching the housing. If the ground line 10 is not provided or if it is arranged in an unsatisfactory way, however, an accident of the points P and Q contacting each other would cause a very high potential (such as 2 kv) on the housing 6, so that a human being touching it would be killed by electric shock. The trouble is thus that the ground line 10 would become indispensable and it would be necessary to guarantee a sufficiently low impedance in the installation work. In other words, the absolute necessity of grounding work for conventional microwave ovens has led to troublesome installation and relocation after installation, thereby making the operation thereof inconvenient.
In the case where the power cord of a microwave oven is a three-terminal cord including a ground terminal, accurate grounding of the indoor wirings could secure safety. Inconveniently, however, a three-terminal power cord is high in cost and the microwave oven can be used only at a place in the home where accurate grounding has been provided for indoor wiring.